Amphiphiles micellar aggregation

Advantage can be taken of the living polymer principle to prepare a wide range of highly controlled-structure polymers that are amphiphilic. The eopolymers of this type that have been studied most are block and graft copolymers of styrene and alkylene oxides. These polymers can form amphiphilic micellar aggregates (168) and polymers with unique phase-transfer activity (169). 

In aqueous solution, amphiphilic molecules aggregate into micelles above the critical micelle concentration. Such solutions have been the object of research for many years, with special interest in shape and size of these micellar aggregates [37]. Size and shape (spherical, wormlike, or disklike micelles) depend strongly on the molecular structure of the amphiphilic molecule. 

Fig. 3.1. The relative amount of amphiphile fm that is incorporated into the micellar aggregate at an infinitesimal increase in the total concentration S as a function of total amphiphile concentration, fm = 1 -, calculated from the mass action law model (Eq. 3.2) with n = 10...

The preceding discussion has been confined to two-component systems, amphiphile-water. In a large number of cases of practical importance one adds one (or more) additional component(s). Depending on the nature of the additive one can recognize different effects. If it is an amphiphile it is usually found that the micelles which form in the solution are of mixed composition. Under the assumption that the amphiphiles mix ideally in the micellar aggregate, Shinoda177 has derived expressions for the effective CMC of the amphiphile mixture. For nonionics... 

The methods mentioned above do not measure the micellar aggregation number itself but rather some micellar size. A direct determination of aggregation numbers can be performed by analysing some physico-chemical parameter in terms of the equilibria involved equilibrium analyses on the basis of potentiometric data have been pioneered by Danielsson and co-workers who studied short-chain, not typically micelle-forming, amphiphiles in the presence of added electrolyte (see above). 

Assume that the micellar aggregate is spherical (radius rm) with the ionic groups of the amphiphile at the surface. Due to the dynamic nature of the micelle the... 

So far, micelles and vesicles of amphiphilic block copolymers with two different blocks have been described. In this section the work on amphiphilic block copolymers and block copolyampholytes composed of three different blocks will be reviewed. Much less work has been carried out on these systems and there are less systematic studies available. Focus will be laid on block copolymers with at least one polyelectrolyte block. While in the case of amphiphilic diblock copolymers questions like the influence of block lengths on the size of micellar aggregates have been studied in great detail, in ternary block copolyampholytes other properties have attracted greater interest, such as the influence of the block sequence on the solution properties and aggregate formation. 

Additives are usually amphiphilic in nature, and thus are either ionic or neutral surfactants or even polymers. The role of surfactants in solvent extraction is ambiguous. Usually, they should be avoided as they lower the interfacial tension, which may lead to emulsion formation in an agitated extractor. However, every metal-loaded ion exchanger is amphiphilic, and can adsorb at the interface or aggregate in the bulk phase. This occurrence is well known with sodium or other metals [17], and above a critical surfactant concentration (cmc, critical micelle concentration) micellar aggregates are formed. A dimensionless geometric parameter is decisive for the structure of the associates, according to Fig. 10.6 ... 

The size distribution of micellar aggregates Ng/F is plotted against the aggregation number, g, for an amphiphile with an octyl hydrocarbon tail and for a 2 X 1G4 cal A2t molsb (Figure 1). Equation 19 leads to wjt = 3.88. For < crit, the size distribution is a monotonic decreasing function of g. At = CT t, the size distribution function has ah inflection point. At > mt, the size distribution function has two extrema. It can be seen that if increases both the number and the average size of the micellar aggregates increase. 

The acid-base properties of the amphiphilic ligands change in the presence of micellar aggregates due to the well known partition equilibrium of both the acid and anionic form. A continuous increase in the apparent pK was observed with increasing concentration of mi-cellized surfactant (see Table I). 

Another possibility for azo-polysiloxane applications is to obtain photo-sensible micelles [15-18]. The interest for this application is explained by the possibility to use polymeric micellar aggregates for controlled release of substances such as drugs [19, 20]. There are few reports on the use of light as an external stimulus for small amphiphilic molecules by incorporahug the azobeuzeue chromophore iuto surfactant [16] or for light-responsive micellar aggregates formed by amphiphilic block copolymers [17,18]. 

The first self-assembling block copolymers were PS-fe-PMPS- -PS synthesised by Matyjaszewski and Moller. They observed micellar aggregates by ATM after casting dilute dioxane solutions (a solvent selective for the PS block) of the copolymer. The observed micelles were taken to have internal PMPS cores and were measured at 25-30nm in diameter [73], The hrst self-assembling amphiphilic polysilane block copolymers to be investigated was the PMPS-PEO multi-block copolymer with normal distribution PMPS blocks and uniform low polydispersity PEO blocks. After dialysis aqueous dispersions of this copolymer formed micellar as well as vesicular structures [78, 79] as shown in Eig. 19. 

For low molecular mass amphiphiles, hydrophobic interactions and surface effects determine the critical concentration at which micellar aggregates are favored over the molecularly dispersed amphiphilic solutes. For polysurfactants, however, the amphiphiles are linked together and the dynamic exchange of associated and non-associated amphiphilic monomer units is prevented. Consequently the micelle formation does not only depends on the hydrophilic/ hydrophobic balance of the monomer... 

Micellar aggregation of the C6oC(COO )2 derivative in water, a consequence of its amphiphilic structure (i.e., hydrophobic fullerene core and hydrophilic carbox-ylates), is responsible for the lack of reduction [89, 90], These clusters are considered to contain an inward oriented hydrophobic fullerene moiety and a hydrophilic layer of carboxylate head groups, which prevent the negatively charged electrons from approaching the fullerene core. 

Figure 3.8 An SDS-dissolved copper porphyrin gives the ESR spectrum a) of a monomer, whereas the same amphiphilic porphyrin in water produces b) the spectrum of a micellar aggregate. ...

The modulation of the coordination to the transition metal has not necessarily positive implications on the reactivity. For instance, we observed [50] that the copper(II) complex (8) of tetramethyl-l,2-diaminoethane catalyzes the hydrolysis of the phosphoric acid triester PNPDPP via an electrophilic mechanism which involves the pseudointramolecular attack of deprotonated water, as illustrated in (9). The electrophilic mechanism contribution to the hydrolytic process totally disappears in micellar aggregates made of the amphiphilic complex (10). Clearly, micellization does not allow the P O group of the substrate to interact with the metal ion. This could be a result of steric constraint of the substrate when bound to the micelle and/or the formation of binuclear dihydroxy complexes, like (11), in the aggregate. So, in spite of the quite large rate accelerations observed [51] in the cleavage of PNPDPP in metallomicelles made of the amphiphilic complex (10), the second-order rate constant [allowing for the difference in pXa of the H2O molecules bound to copper(II) in micelles and monomers] is higher for (8) than for (10) (k > 250). 

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